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Chapter 20: Problem 3

How do the acidities of the aqueous solutions of the alkaline earth metal ions \(\left(\mathrm{M}^{2+}\right)\) change in going down the group?

Short Answer

Expert verified
The acidity of aqueous solutions of alkaline earth metal ions (M^2+) decreases as we move down the group in the periodic table. This is because the increasing size of the metal ions leads to a decrease in their ability to polarize the electron cloud of the surrounding water molecules within hydrated complexes, making them more stable and thus less likely to donate H+ ions to the solution.

Step by step solution

01

Understand Acidity and Alkaline Earth Metals

The acidity of a solution refers to its ability to donate H+ ions, as described by the Brønsted-Lowry theory. Aqueous solutions of alkaline earth metal ions (M^2+) are formed when alkaline earth metals react with water, releasing metal ions and hydroxide ions (OH-). Alkaline earth metals are found in Group 2 of the periodic table and include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).
02

Look for Trends in Alkaline Earth Metals

As we move down the group, the atomic size of the alkaline earth metals increases and the ionization energy (the energy required to remove an electron from an atom) decreases. These changes result in a stronger tendency for the metals to lose electrons and form cations (M^2+), which increases the dissociation of the metal ions in water.
03

Understand the Relationship between Acidity and Metal Ion Properties

When a metal ion with a 2+ charge (such as an alkaline earth metal ion) dissolves in water, it attracts water molecules and forms a hydrated complex: \[ \mathrm{M}^{2+} + \textit{n} \, \mathrm{H_2O} \longrightarrow \mathrm{M(H_2O)_n}^{2+} \] The water molecules in the hydrated complex can donate H+ ions to the surrounding solution, which increases its acidity. The acidity of the solution depends on the stability of the hydrated complex: the more stable the complex, the less H+ ions will be donated and the lower the acidity.
04

Analyze the Stability of the Hydrated Complexes

As we move down the group, the size of the alkaline earth metal ions increases, and their tendency to polarize (distort) the electron cloud of the surrounding water molecules decreases. Consequently, the stability of the hydrated complexes increases, as the metal ions are less able to destabilize the water molecules and promote the dissociation of H+ ions. This results in a decrease in acidity of the aqueous solutions. Therefore, we can conclude that the acidity of the aqueous solutions of the alkaline earth metal ions decreases as we move down the group in the periodic table.

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